(1) Field of the Invention
The present invention relates to an image processing apparatus, an image processing method and an image processing program, and in relation to a Compact PDF file, particularly relates to a technique to compress an image with a high compression ratio in such a manner that a time required for displaying and printing a decompressed image is reduced.
(2) Description of the Related Art
In recent years, various compression methods for reducing a file size have been used for digitizing documents. Especially, a Compact PDF (Portable Document Format) method is promising as a method realizing a high compression ratio with maintaining a high quality of a decompressed image.
In the Compact PDF method, an image layer is formed for each object, such as a character and a figure included in an image. Image processing and image compression is performed in accordance with the type of the object included in the image layer. As a result, the Compact PDF method is capable of achieving both a high image quality and a high compression ratio.
FIG. 1 shows an example of the image layer. Image layers are respectively formed for objects such as characters and figures, which are included in an image on the left in FIG. 1.
However, each image layer is required to individually hold attribute information indicating the position of the object, the type of compression and so on. Therefore, if many image layers are included, the total data size of the attribute information is large. To reduce the total data size of the attribute information, a conventional art integrates image layers sharing a common attribute (See U. S. Patent Application Publication No. 2002/0037100 A1).
In FIG. 1, in the case where attribute information indicating that the character color is black is given to the image layer 1201 and attribute information indicating that the character color is red is given to the image layers 1202 and 1203, the image layers 1202 and 1203 will be integrated to form an image layer 1204.
In this way, the conventional art is capable of reducing the number of the image layers and reducing the total data size of the attribute information.
If such a conventional art is employed, in the case where objects to be integrated are away from each other, a blank space is added to the integrated image layer and the number of pixels included in the integrated image layer greatly increases (e.g. the image layer 1204 in FIG. 1). The blank space can be compressed with a high compression ratio. Therefore, the blank space does not affect the size of the compressed file.
However, to decompress the file for displaying and printing the image, it is necessary to decompress the blank space as well. This lengthens the time required for the decompression. Especially, in the case where objects are diagonally positioned so that they are away from each other, the blank space is large and the required time is greatly increased.